The objective of this research is to establish a genetic basis of male recombination-mutator system (Mr-system of Drosophila melanogaster, and to obtain better understanding of the roles of mutation and crossing over in evolution. Specifically, following points will be studied more intensively. 1. Mapping of the elements in this system. One major element, Mr, was mapped successfully to the centromere region of chromosome 2. Similar procedure will be applied to the Mr(like) element in chromosome 3 -- is it also located at the centromere region? Formal genetics of this third-chromosome element will be established. It was recently found that some of the X-chromosomes carry suppressor of Mr. Formal genetics studies will be made for this suppressor element also. 2. It is well established that, in the Mr system, recombination frequency, distorted segregation frequency and fertility in males are correlated. As a next step, cytological observations (spermato- and spermio-genesis) will be conducted to see in what stage, and how the distortion takes place. This observation will give some information about the mechanism and procedure in the Male recombination system. 3. Population genetics. We now have basic information of the Mr-system sufficient to start to think of the evolutionary implications of this system. Two population cages were constructed a few years ago, and field collections from natural populations provide materials for this study. Effort will be paid to study the relationship between Mr and suppressors (modifier in general) in populations. BIBLIOGRAPHIC REFERENCES: Slatko, B. and Y. Hiraizumi 1975. Elements causing male crossing over in Drosophila melanogaster. Genetics 81: 313-324. Matthews, K. and Y. Hiraizumi 1976. Frequency of male recombination element (Mr) in a South Texas population of Drosophila melanogaster. Genetics 83: S48. Abstract.